The present invention relates to a process for producing acetophenone-modified phenolic resin, and its object is to provide a modified phenolic resin which is inexpensive and is provided with a satisfactory thermosetting property.
Thermosetting phenolic resins are generally obtained as products of reaction between phenols such as phenol or cresol and aldehydes such as formaldehyde or acetaldehyde in an acidic or alkaline condition. Structurally the phenolic resins are classified as novolak resins and resol resins. Novolak resins are obtained by a reaction of phenol or cresol with formaldehyde generally conducted in the presence of an acidic catalyst, wherein the reaction product has a linear structure as represented by the following reaction formulas: ##STR1## As shown above the reaction proceeds by the repetition of addition and condensation to provide a resinous product, which is a mixture of di- to deca-condensate of phenolic compounds (m.w. 200-1300) with an average of hexa-condensate of phenolic compounds, which is solid at room temperature, soluble in alcohol or other solvents and easily melted by heating. Novolak phenolic resins are not hardened by heating and require, for hardening, the use of a hardener which is usually hexamethylenetetramine (hereinafter referred to as hexamine). On the other hand resol resins are obtained by a reaction of phenol or cresol with formaldehyde in an alkaline condition according to the following formulas: ##STR2## The reaction is initiated by the addition of formaldehyde to phenol and proceeds by condensation and addition reactions to obtain a mixture of di- to pentadecan-condensate phenolic compounds. In contrast to novolak resins, resol resins are hardened by heating due to the presence of crosslinkable methylol radicals at the arbitrary terminals of molecules.
Phenols, which are the principal raw material for phenolic resins, can be produced by coal tar fractionation or by processes starting from petroleum such as the benzene sulfonation process, the cumene process or the chlorobenzene process. Most of the present phenols are synthetically obtained by the latter processes using petroleum derived products. It is usual that phenols of a relatively high purity are generally used for the production of phenolic resins. However, the significant surge of petroleum price in recent years has resulted in an increase in the price of phenols derived from petroleum, and the resulting increase in the manufacturing cost of phenolic resins has reached a situation wherein phenolic resins, which have been accepted as inexpensive and durable thermosetting resins, are no longer applicable for certain purposes because of economic consideration. In order to reduce the manufacturing cost of the phenolic resins, therefore, it has been contemplated to replace a part of phenols with another inexpensive material, and such approach is undoubtedly desirable also from the standpoint of effective utilization of petroleum resources.
As a result of exploratory research based on such economic and social background, we have succeeded in obtaining inexpensive and still satisfactorily thermosetting phenolic resins by employing a scarcely utilized by-product obtained in phenol production by the cumene process which principally consists of acetophenone, and reacting it in a particular manner during the course of phenolic resin preparation. The by-product obtained in phenol production by the cumene process is composed of about 70 to 90 weight percent of acetophenone, about 5 to 10 weight percent of phenol and trace amounts of cresol, xylenol, .alpha.-methylstyrene, dimethylphenylcarbinol, etc. The reaction of acetophenone with formaldehyde is reported in a prior reference (S. G. Morgan, Chemistry and Industry, 1938), but no report of it concerning the modification of phenolic resins is found.